1D Compression

Loaded in one dimension, as in an oedometer, a soil's void ratio falls with the logarithm of the vertical effective stress. Below its preconsolidation stress σ′_pc, the largest stress it has carried, it follows a stiff recompression line of slope κ. Loaded past σ′_pc, it yields onto the virgin compression line of slope λ, and σ′_pc moves up to the new past maximum. Unloading from the peak follows a swelling line of slope κ, so the soil keeps most of the compression and is left overconsolidated.

What you are seeing

• The chart. Void ratio against vertical effective stress, on a logarithmic axis, so every line is straight. The dashed black line is the virgin compression line; the dotted grey line is the initial recompression line, up to the initial σ′_pc (dotted grey vertical line).
• The path. Loading (solid blue) runs from the initial stress to the peak, along the recompression line and then, if the peak passes σ′_pc, along the virgin line. Unloading (solid orange) runs back from the peak to the final stress, on the current unload–reload line (dash-dot orange). If the soil yielded, the dashed orange vertical line marks the new σ′_pc. The black points are the initial, peak and final states.
• The outputs. The overconsolidation ratios and void ratios of those states, and a note on whether the soil yielded.